Modular, high energy, widely-tunable ultrafast fiber source

ABSTRACT

A modular, compact and widely tunable laser system for the efficient generation of high peak and high average power ultrashort pulses. Peak power handling capability of fiber amplifiers is expanded by using optimized pulse shapes, as well as dispersively broadened pulses. Dispersive pulse stretching in the presence of self-phase modulation and gain results in the formation of high-power parabolic pulses. To ensure a wide tunability of the whole system, Raman-shifting of the compact sources of ultrashort pulses in conjunction with frequency-conversion in nonlinear optical crystals can be implemented, or an Anti-Stokes fiber in conjunction with fiber amplifiers and Raman-shifters are used. Positive dispersion optical amplifiers are used to improve transmission characteristics. An optical communication system utilizes a Raman amplifier fiber pumped by a train of Raman-shifted, wavelength-tunable pump pulses, to thereby amplify an optical signal which counterpropagates within the Raman amplifier fiber with respect to the pump pulses.

This is a Divisional Application of Ser. No. 13/187,566, filed Jul. 21,2011, which is a Continuation application of Ser. No. 12/608,602, filedOct. 29, 2009, now issued as U.S. Pat. No. 8,031,396 on Oct. 4, 2011,which is a Divisional application of application Ser. No. 11/643,765,filed Dec. 22, 2006, now issued as U.S. Pat. No. 7,688,499 on Mar. 30,2010, which is a Continuation of application Ser. No. 11/074,765, filedMar. 9, 2005, now issued as U.S. Pat. No. 7,167,300 on Jan. 23, 2007,which is a Divisional Application of application Ser. No. 09/576,772,filed May 23, 2000, now issued as U.S. Pat. No. 6,885,683 on Apr. 26,2005. The disclosures of which are incorporated herein by reference intheir entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to wavelength-tunable, compact, modular andefficient sources of high-power ultrashort laser pulses, which are anessential ingredient in the commercial use of ultrafast lasertechnology.

2. Description of the Related Art

Fiber lasers have long been known to provide an effective medium for thegeneration of ultrashort pulses, though so far such systems have mainlybeen based on chirped pulse amplification using chirped Bragg gratings,with limited options for wavelength tunability and limitations in theminimal achievable pulse width (A. Galvanauskas and M. E. Fermann,‘Optical Pulse Amplification Using Chirped Bragg Gratings,’ U.S. Pat.No. 5,499,134). Chirped Bragg gratings have indeed been developed intowidely available devices, and the chirp inside the Bragg gratings can bedesigned to be linear or even nonlinear to compensate any order ofdispersion in a chirped pulse amplification system (A. Galvanauskas etal., ‘Hybrid Short-Pulse Amplifiers with Phase-Mismatch CompensatedPulse Stretchers and Compressors’, U.S. Pat. No. 5,847,863), which isimportant for the generation of bandwidth limited pulses, i.e., theshortest possible pulses for a given spectral pulse bandwidth.

To maximize the power and energy limitations of optical fibers, the useof chirped pulse amplification is clearly desirable, though at the sametime the demands for system integration (Bragg gratings need to beoperated in reflection rather than in transmission to provide thehighest possible dispersion) may render the use of such standard chirpedpulse amplification systems impractical. As an alternative to chirpedpulse amplification, the amplification of high-power pulses inmulti-mode fiber amplifiers has been suggested (M. E. Fermann and D.Harter, ‘Single-mode Amplifiers and Compressors Based on Multi-modeOptical Fibers’, U.S. Pat. No. 5,818,630). As yet another alternative tochirped pulse amplification the use of soliton Raman compression infiber amplifiers, or, generally, the use of pulse compression insidenonlinear fiber amplifiers was proposed (M. E. Fermann, A. Galvanauskasand D. Harter, ‘Apparatus and Method for the Generation of High-powerFemtosecond Pulses from a Fiber Amplifier’, U.S. Pat. No. 5,880,877).

Clearly the use of multi-mode fibers can be combined with chirped pulseamplification and soliton Raman compression to further improve theperformance of such systems. However, to date no methods for controllingthe pulse-shape for a further optimization of the overall systemperformance have been described. Equally, the use of self-phasemodulation in the stretcher part of such chirped pulse amplificationsystems has not been suggested.

Moreover, as a compromise between system compactness and high-energycapability, the use of a fiber dispersive delay line in conjunction witha bulk optic compressor can be advantageous, providing at least partialintegration of a high-energy fiber laser system [M. E. Fermann A.Galvanauskas and D. Harter: ‘All fiber source of 100 nJ sub-picosecondpulses’, Appl. Phys. Lett., vol. 64, 1994, pp. 1315-1317]. However, todate no effective methods for controlling higher-order 3rd and 4th orderdispersion in such stretcher and compressor combinations for there-compression of the pulses to near their bandwidth limit have beendeveloped.

As an alternative to chirped pulse amplification, it was also previouslysuggested that efficient pulse compression can be obtained by usinghigh-gain positive dispersion (non-soliton supporting) silica-basedsingle-mode erbium amplifiers in combination with bulk prism compressors(K. Tamura and M. Nakazawa, “Pulse Compression by Nonlinear PulseEvolution with Reduced Optical Wave Breaking in Erbium-Doped FiberAmplifiers,” Opt. Lett., Vol. 21, p. 68 (1996)). However, the use ofthis technique in conjunction with silica-based erbium amplifiers isproblematic, because the requirement for positive dispersion limits thefiber core size to around 5 μm, otherwise, negative material dispersiondominates over positive waveguide dispersion, producing an overallnegative fiber dispersion. Equally, silica-based multi-mode fibers havenegative dispersion at erbium amplifier wavelengths, preventing theiruse in efficient pulse compression. Thus, the limited core size ofpositive dispersion erbium amplifiers greatly reduces the achievablepulse energy.

Moreover, it was not shown by Tamura et al. how to generate additionalspectral broadening and pulse amplification after the one erbiumamplifier. Equally, it was not taught by Tamura et al. how to optimizethe performance of the prism pulse compressor to compensate for thedispersion of the erbium amplifier.

As another alternative to chirped pulse amplification, the use of anon-amplifying optical fiber in conjunction with a bulk gratingcompressor was suggested (D. Grischkowsky et al. and J. Kafka et al.,U.S. Pat. No. 4,750,809). However, since there is no gain in such asystem, high pulse energies have to be coupled into the nonlinearoptical element in order to obtain a high output power, greatly reducingthe peak power capability of the system. Moreover, no means forcompensating for higher-order dispersion in such an optical arrangementwas discussed, greatly limiting the practicability of this approach. Inaddition, without control of the pulse shape at the input to such asystem, spectral broadening with a linear chirp can only be obtained forvery limited input powers. Control of the input pulse shape was notdiscussed by Kafka et al. Equally, to obtain the shortest possiblepulses in conjunction with a bulk grating compressor, the control of 2ndand 3rd order dispersion in such a nonlinear optical element isrequired, which was also not discussed by Kafka et al.

Compensation for chromatic dispersion in a (low-power) lightwave signalusing the chromatic dispersion in another (dispersion-compensating)waveguide element was introduced to optimize the performance oftelecommunication systems (C. D. Poole, ‘Apparatus of compensatingchromatic dispersion in optical fibers’, U.S. Pat. No. 5,185,827).However, for high-power pulse sources, self-phase modulation introducedby a dispersion-compensating waveguide element prevents their effectiveuse. Moreover, the system discussed by Poole only operates inconjunction with mode-converters and/or rare-earth-doped fiber foreither selectively absorbing a higher-order spatial mode in thedispersion-compensating waveguide element or selectively amplifying thefundamental mode in the dispersion-compensating waveguide element. Nomeans were taught for compensating for the dispersion of high-poweroptical pulses in the presence of self-phase modulation, and no means ofimplementing a dispersion-compensating waveguide without mode-converterswere suggested.

As an alternative to the use of mode-converters and higher-order modes,fibers with W-style refractive index profiles are known (B. J. Ainslieand C. R. Day, ‘A review of single-mode fibers with modified dispersioncharacteristics’; J. Lightwave Techn., vol. LT-4, No. 8, pp. 967-979,1988). However, the use of such fiber designs in high-power fiberchirped pulse amplification systems has not been discussed.

To maximize the efficiency of ultrafast fiber amplifiers, the use of Ybfiber amplifiers has been suggested (D. T. Walton, J. Nees and G.Mourou, “Broad-bandwidth pulse amplification to the 10 μJ level in anytterbium-doped germanosilicate fiber,” Opt. Lett., vol. 21, no. 14, pp.1061 (1996)), though the work by Walton et al., employed an Argon-laserpumped Ti:sapphire laser for excitation of the Yb-doped fibers as wellas a modelocked Ti:sapphire laser as a source of signal pulses, which isextremely inefficient and clearly incompatible with a compact set-up.Moreover, no means for controlling the phase of the optical pulses inthe amplification process were suggested, i.e., 100 fs pulses from theTi:sapphire laser were directly coupled to the Yb amplifier through a1.6 km long single-mode fiber dispersive delay line, which produceslarge phase distortions due to higher-order dispersion, greatly limitingthe applicability of the system to ultrafast applications. Rather, toinduce high-quality high-power parabolic pulse formation inside the Ybamplifier, seed pulses in the range from 200-400 fs would be preferablefor an Yb amplifier length of a few meters. The use of a single-modeYb-doped fiber amplifier by Walton et al. further greatly limited theenergy and power limits of the Yb amplifier. The use of a multi-modeYb-doped fiber was suggested in U.S. application Ser. No. 09/317,221,the contents of which are hereby incorporated herein by reference,though a compact ultrashort pulse source compatible with Yb amplifiersremained elusive.

A widely tunable pulsed Yb-fiber laser was recently describedincorporating an active optical modulation scheme (J. Porta et al.,‘Environmentally stable picosecond ytterbium fiber laser with a broadtuning range’, Opt. Lett., vol. 23, pp. 615-617 (1998). Though thisfiber laser offered a tuning range approximately within the gainbandwidth of Yb, application of the laser to ultrafast optics is limiteddue to the relatively long pulses generated by the laser. Generally,actively modelocked lasers produce longer pulses than passivelymodelocked lasers, and in this present case the generated pulsebandwidth was only 0.25 nm with a minimal pulse width of 5 ps.

Widely wavelength-tunable fiber laser sources were recently describedusing Raman-shifting in conjunction with frequency-conversion in anonlinear crystal. (See M. E. Fermann et al., U.S. Pat. No. 5,880,877and N. Nizhizawa and T. Goto, “Simultaneous Generation of WavelengthTunable Two-Colored Femtosecond Solution Pulses Using Optical Fibers,”Photonics Techn. Lett., vol. 11, no. 4, pp 421-423). Essentiallyspatially invariant fiber Raman shifters were suggested, resulting inlimited wavelength tunability of 300-400 nm (see Nizhizawa et al.).Moreover, no method was known for minimizing the noise of such a highlynonlinear system based on the successive application of Raman shiftingand nonlinear frequency conversion in a nonlinear optical crystal.Further, the system described by Nizhizawa et al. relied on a relativelycomplex low power polarization controlled erbium fiber oscillatoramplified in an additional polarization controlled erbium fiberamplifier for seeding the Raman shifter. Moreover, no method wasdescribed that allowed Raman-shifting of the frequency-doubled outputfrom an Er fiber laser.

A Raman shifter seeded directly with the pulses from a high-power fiberoscillator or the frequency-converted pulses from a high-power fiberoscillator would clearly be preferable. Such fiber oscillators wererecently described using multi-mode optical fibers (M. E. Fermann,‘Technique for mode-locking of multi-mode fibers and the construction ofcompact high-power fiber laser pulse sources’, U.S. Ser. No.09/199,728). However, to date no methods for frequency-converting suchoscillators with the subsequent use of Raman-shifting have beendemonstrated.

SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to introduce amodular, compact, widely-tunable, high peak and high average power, lownoise ultrafast fiber amplification laser system.

It is a further object of the invention to ensure modularity of thesystem by employing a variety of easily interchangeable optical systems,such as 1) short pulse seed sources, 2) wide bandwidth fiber amplifiers,3) dispersive pulse stretching elements, 4) dispersive pulse compressionelements, 5) nonlinear frequency conversion elements and 6) opticalcomponents for fiber delivery. In addition, any of the suggested modulescan be comprised of a subset of interchangeable optical systems.

It is a further object of the invention to ensure system compactness byemploying efficient fiber amplifiers, directly or indirectly pumped bydiode lasers as well as highly integrated dispersive delay lines. Thehigh peak power capability of the fiber amplifiers is greatly expandedby using parabolic or other optimized pulse shapes. In conjunction withself-phase modulation, parabolic pulses allow for the generation oflarge-bandwidth high-peak power pulses, as well as for well-controlleddispersive pulse stretching. High power parabolic pulses are generatedin high-gain single or multi-mode fiber amplifiers operating atwavelengths where the fiber material dispersion is positive.

Parabolic pulses can be delivered or transmitted along substantial fiberlengths even in the presence of self-phase modulation or generalKen-effect type optical nonlinearities, while incurring only asubstantially linear pulse chirp. At the end of such fiber delivery orfiber transmission lines, the pulses can be compressed to approximatelytheir bandwidth limit.

Further, the high energy capability of fiber amplifiers is greatlyexpanded by using chirped pulse amplification in conjunction withparabolic pulses or other optimized pulse shapes, which allow thetoleration of large amounts of self-phase modulation without adegradation of pulse quality. Highly integrated chirped pulseamplification systems are constructed without compromising thehigh-energy capabilities of optical fibers by using fiber-based pulsestretchers in conjunction with bulk-optic pulse compressors (or lownonlinearity Bragg gratings) or periodically poled nonlinear crystals,which combine pulse compression with frequency-conversion.

The dispersion in the fiber pulse stretcher and bulk optic compressor ismatched to quartic order in phase by implementing fiber pulse stretcherswith adjustable 2nd, 3rd and 4th order dispersion. Adjustablehigher-order dispersion can be obtained by using high numerical aperturesingle-mode fibers with optimized refractive index profiles by itself orby using standard step-index high numerical aperture fibers inconjunction with linearly chirped fiber gratings. Alternatively,higher-order dispersion can be controlled by using the dispersiveproperties of the higher-order mode in a high numerical aperturefew-moded fiber, by using nonlinearly chirped fiber gratings or by usinglinearly chirped fiber gratings in conjunction with transmissive fibergratings. Adjustable 4th order dispersion can be obtained by controllingthe chirp in fiber Bragg gratings, transmissive fiber gratings and byusing fibers with different ratios of 2^(nd), 3^(rd) and 4^(th) orderdispersion. Equally, higher-order dispersion control can be obtained byusing periodically poled nonlinear crystals.

The fiber amplifiers are seeded by short pulse laser sources, preferablyin the form of short pulse fiber sources. For the case of Yb fiberamplifiers, Raman-shifted and frequency doubled short pulse Er fiberlaser sources can be implemented as widely tunable seed sources. Tominimize the noise of frequency conversion from the 1.5 μm to the 1.0 μmregime, self-limiting Raman-shifting of the Er fiber laser pulse sourcecan be used. Alternatively, the noise of the nonlinear frequencyconversion process can be minimized by implementing self-limitingfrequency-doubling, where the center wavelength of the tuning curve ofthe doubling crystal is shorter than the center wavelength of theRaman-shifted pulses.

The process of Raman-shifting and frequency-doubling can also beinverted, where an Er fiber laser is first frequency-doubled andsubsequently Raman-shifted in an optimized fiber providingsoliton-supporting dispersion for wavelengths around 800 nm and higherto produce a seed source for the 1 μm wavelength regime.

As an alternative low-complexity seed source for an Yb amplifier, amodelocked Yb fiber laser can be used. The fiber laser can be designedto produce strongly chirped pulses and an optical filter can beincorporated to select near bandwidth-limited seed pulses for the Ybamplifier.

Since parabolic pulses can be transmitted along substantial fiberlength, they can also be used in fiber optic communication systems. Inthis, parabolic pulses can be transmitted that were generated by anexternal pulse source. Alternatively, parabolic pulses can also begenerated in the transmission process. For the latter case, thedeleterious effect of optical nonlinearities in the transmission systemare generally minimized by implementing long, distributed, positivedispersion optical amplifiers. Such amplifiers can have lengths of atleast 10 km and a gain of less than 10 dB/km. The total gain peramplifier should exceed 10 dB, in order to exploit the onset ofparabolic pulse formation for a minimization of the deleterious effectof optical nonlinearities. Chirp compensation along the transmissionlines can be conveniently implemented using chirped fiber Bragg gratingsalong the fiber transmission line, and also at the end of thetransmission line. Optical bandwidth filters can further be implementedfor bandwidth control of the transmitted pulses.

Wavelength-tunable pulse sources based on Raman-shifting of short pulsesin optical fibers are useful in itself for many applications, forexample in spectroscopy. However, a very attractive device can beconstructed by the application of Raman-shifting to the construction ofwavelength-tunable fiber Raman amplifiers for telecommunication systems.In this wavelength-tunable system, Raman-shifted pump pulses provideRaman gain for a tunable wavelength range, which is red-shifted withrespect to the pump pulses. Moreover, the shape of the Raman gainspectrum can be controlled by modulating the Raman-shifted pump pulses.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the followingdescription in conjunction with the accompanying drawings, wherein:

FIG. 1 is an illustration of a modular, compact, tunable system forgenerating high peak and high average power ultrashort laser pulses inaccordance with the present invention;

FIG. 2 is an illustration of an embodiment of a Seed Module (SM) for usein the present invention;

FIG. 3 is a diagram graphically illustrating the relationship betweenthe average frequency-doubled power and wavelength which are output at agiven range of input power according to one embodiment of the presentinvention.

FIG. 4 is an illustration of an embodiment of a Pulse Compressor Module(PCM) for use with the present invention;

FIG. 5 is an illustration of an embodiment of a Pulse Stretcher Module(PSM) for use with the present invention;

FIG. 6 is an illustration of a second embodiment of a Seed Module (SM)for use with the present invention;

FIG. 7 is an illustration of a third embodiment of a Seed Module (SM)for use with the present invention;

FIG. 8 is an illustration of a fourth embodiment of a Seed Module (SM)for use with the present invention;

FIG. 9 is an illustration of a fifth embodiment of a Seed Module (SM)for use with the present invention;

FIG. 10 is an illustration of an embodiment of the present invention inwhich a Fiber Delivery Module (FDM) is added to the embodiment of theinvention shown in FIG. 1;

FIG. 11 is an illustration of an embodiment of a Fiber Delivery Module(FDM) for use with the present invention;

FIG. 12 is an illustration of a second embodiment of a Pulse StretcherModule (PSM) for use with the present invention;

FIG. 13 is an illustration of a third embodiment of a Pulse StretcherModule (PSM) for use with the present invention;

FIG. 14 is an illustration of an embodiment of the present invention inwhich pulse picking elements and additional amplification stages areadded.

FIG. 15 is an illustration of another embodiment of the presentinvention where a fiber amplifier is operated with at least one forwardand one backward pass, in combination with optical modulators such aspulse picking elements.

FIG. 16 is an illustration of another embodiment of the presentinvention in the context of an optical communication system.

FIG. 17 is an illustration of another embodiment of the present systemin the context of a wavelength-tunable Raman amplifier fortelecommunications.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A generalized illustration of the system of the invention is shown inFIG. 1. The pulses generated in a laser seed source 1 (seed module; SM)are coupled into a pulse stretcher module 2 (PSM), where they aredispersively stretched in time. The stretched pulses are subsequentlycoupled into the fundamental mode of a cladding-pumped Yb fiberamplifier 3 (amplifier module, AM1), where the pulses are amplified byat least a factor of 10. Finally, the pulses are coupled into a pulsecompressor module 4 (PCM), where they are temporally compressed back toapproximately the bandwidth limit.

The embodiment shown in FIG. 1 is modular and consists of the foursub-systems; the SM 1, PSM 2, AM 1 3 and PCM 4. The sub-systems can beused independently as well as in different configurations, as describedin the alternative embodiments.

In the following, discussion is restricted to the SM-PSM-AM1-PCM system.The SM 1 preferably comprises a femtosecond pulse source (seed source5). The PSM preferably comprises a length of fiber 6, where couplingbetween the SM and the PSM is preferably obtained by fusion splicing.The output of the PSM is preferably injected into the fundamental modeof the Yb amplifier 7 inside the AM 1 module 3. Coupling can beperformed by either fusion splicing, a fiber coupler or a bulk-opticimaging system between PSM 2 and the fiber amplifier 7. All fibers arepreferably selected to be polarization maintaining. The PCM 4 preferablyconsists of a dispersive delay line constructed from one or two bulkoptic diffraction gratings for reasons of compactness. Alternatively, anumber of bulk optic prisms and Bragg gratings can be used inside thePCM 4. Coupling to the PCM 4 can be performed by a bulk optic lenssystem as represented by the single lens 8 in FIG. 1. In the case of aPCM that contains fiber Bragg gratings, a fiber pig-tail can be used forcoupling to the PCM.

As an example of a femtosecond laser seed source, a Raman-shifted,frequency-doubled Er fiber laser is shown within an SM 1 b in FIG. 2.The femtosecond fiber laser 9 can be a commercial high energy solitonsource (IMRA America, Inc., Femtolite B-60™) delivering ≈200 fs pulsesat a wavelength of 1.57 μm and a pulse energy of 1 nJ at a repetitionrate of 50 MHz.

For optimum Raman-shifting from 1.5 μm to the 2.1 μm wavelength region,a reduction in the core diameter (tapering) along the length of thepolarization maintaining Raman-shifting fiber 10 is introduced. Areduction of the core diameter is required to keep the 2nd orderdispersion in the Raman-shifter close to zero (but negative) in thewhole wavelength range from 1.5 to 2.1 μm. By keeping the absolute valueof the 2nd order dispersion small, the pulse width inside the Ramanshifter is minimized, which leads to a maximization of the Ramanfrequency shift (J. P. Gordon, “Theory of the Soliton Self-frequencyShift,” Opt. Lett., 11, 662 (1986)). Without tapering, the Ramanfrequency-shift is typically limited to around 2.00 μm, which even afterfrequency-doubling is not compatible with the gain bandwidth of Yb fiberamplifiers.

In this particular example, a two-stage Raman shifter 10 consisting of30 and 3 m lengths of silica ‘Raman’ fiber (single-mode at 1.56 μm) withcore diameters of 6 and 4 μm respectively, was implemented. Due to theonset of the infrared absorption edge of silica at 2.0 μm, it isbeneficial to increase the rate of tapering towards the end of the Ramanshifter 10. In the present example, conversion efficiencies up to 25%from 1.57 μm to 2.10 μm were obtained. Even better conversionefficiencies can be obtained by using a larger number of fibers withsmoothly varying core diameter, or by implementing a single taperedfiber with smoothly varying core diameter.

Frequency-conversion of the Raman-shifted pulses to the 1.05 μm regioncan be performed by a length of periodically poled LiNbO3 (PPLN) crystal11 with an appropriately selected poling period. (Although throughoutthis specification, the preferable material for frequency conversion isindicated as PPLN, it should be understood that other periodically-poledferroelectric optical materials such as PP lithium tantalate, PPMgO:LiNbO₃, PP KTP, or other periodically poled crystals of the KTPisomorph family can also be advantageously used.) Coupling with the PPLNcrystal 11 occurs through the use of a lens system, represented in FIG.2 by lenses 12. The output of the PPLN crystal 11 is coupled by lenses12 into output fiber 13. Conversion efficiencies as high as 16% can sobe obtained for frequency-doubling of 2.1 μm resulting in a pulse energyup to 40 pJ in the 1 μm wavelength region. The spectral width of thefrequency-converted pulses can be selected by an appropriate choice ofthe length of the PPLN crystal 11; for example a 13 mm long PPLN crystalproduces a bandwidth of 2 nm in the 1.05 μm region corresponding to apulse width of around 800 fs. The generated pulse width is approximatelyproportional to the PPLN crystal length, i.e., a frequency convertedpulse with a 400 fs pulse width requires a PPLN length of 6.5 mm. Thispulse width scaling can be continued until the frequency-converted pulsewidth reaches around 100 fs, where the limited pulse width of 100 fs ofthe Raman-shifted pulses limits further pulse width reduction.

In addition, when the frequency-converted pulse width is substantiallylonger than the pulse width of the Raman-shifted pulses, the widebandwidth of the Raman-pulses can be exploited to allow forwavelength-tuning of the frequency-converted pulses, i.e., efficientfrequency conversion can be obtained for pulses ranging in frequencyfrom 2(ω₁−δω) to 2(ω₁+δω), where 2δω is the spectral width at halfmaximum of the spectrum of the Raman-shifted pulses. Continuouswavelength tuning here is simply performed by tuning the temperature ofthe frequency-conversion crystal 11.

The amplified noise of the Raman-shifter, PPLN-crystal combination isminimized as follows. Self-limiting Raman-shifting of the Er fiber laserpulse source can be used by extending the Raman shift out to larger than2 μm in silica-based optical fiber. For wavelengths longer than 2 μm,the infrared absorption edge of silica starts to significantly attenuatethe pulses, leading to a limitation of the Raman shift and a reductionin amplitude fluctuations, i.e., any increase in pulse energy at 1.5 μmtends to translate to a larger Raman-shift and thus to a greaterabsorption in the 2 μm wavelength region, which thus stabilizes theamplitude of the Raman-shifted pulses in this region.

Alternatively, the noise of the nonlinear frequency conversion processcan be minimized by implementing self-limiting frequency-doubling, wherethe center wavelength of the tuning curve of the doubling crystal isshorter than the center wavelength of the Raman-shifted pulses. Again,any increase in pulse energy in the 1.5 μm region translates into alarger Raman-shift, producing a reduced frequency conversion efficiency,and thus the amplitude of the frequency-doubled pulses is stabilized.Therefore a constant frequency-converted power can be obtained for alarge variation in input power.

This is illustrated in FIG. 3, where the average frequency-convertedpower in the 1 μm wavelength region as a function of average input powerat 1.56 μm is shown. Self-limiting frequency-doubling also ensures thatthe frequency-shifted wavelength in the 1 μm wavelength region isindependent of average input power in the 1.56 μm wavelength region, asalso demonstrated in FIG. 3.

Several options exist for the PSM 2. When a length of fiber 6(stretching fiber) is used as a PSM as shown in FIG. 1, an appropriatedispersive delay line can then be used in the PCM 4 to obtain nearbandwidth-limited pulses from the system. However, when the dispersivedelay line in the PCM 4 consists of bulk diffraction gratings 14 asshown in FIG. 4, a possible problem arises. The ratio of|3^(rd)/2^(nd)|-order dispersion is typically 1-30 times larger indiffraction grating based dispersive delay lines compared to the ratioof |3^(rd)/2^(nd)|-order dispersion in typical step-index optical fibersoperating in the 1 μm wavelength region. Moreover, for standardstep-index fibers with low numerical apertures operating in the 1 μmwavelength regime, the sign of the third-order dispersion in the fiberis the same as in a grating based dispersive delay line. Thus a fiberstretcher in conjunction with a grating-based stretcher does nottypically provide for the compensation of 3^(rd)- and higher-orderdispersion in the system.

For pulse stretching by more than a factor of 10, the control ofthird-order and higher-order dispersion becomes important for optimalpulse compression in the PCM 4. To overcome this problem, the stretcherfiber 6 in the PSM 2 can be replaced with a length of fibers withW-style multi-clad refractive index profiles, i.e., ‘W-fibers’ (B. J.Ainslie et al.) or holey fibers (T. M. Monroe et al., ‘Holey OpticalFibers’ An Efficient Modal Model, J. Lightw. Techn., vol. 17, no. 6, pp.1093-1102). Both W-fibers and holey fibers allow adjustable values of2nd, 3rd and higher-order dispersion. Due to the small core sizepossible in W and holey fibers, larger values of 3rd order dispersionthan in standard single-mode fibers can be obtained. The implementationis similar to the one shown in FIG. 1 and is not separately displayed.The advantage of such systems is that the PSM can work purely intransmission, i.e., it avoids the use of dispersive Bragg gratingsoperating in reflection, and can be spliced into and out of the systemfor different system configurations.

An alternative PSM 2 with adjustable values of 2^(nd), 3^(rd) and 4^(th)order dispersion is shown in FIG. 5. The PSM 20 a is based on theprinciple that conventional step-index optical fibers can produce eitherpositive, zero or negative 3rd order dispersion. The highest amount of3rd order dispersion in a fiber is produced by using its firsthigher-order mode, the LP₁₁ mode near cut-off. In FIG. 5, the 4^(th) and3^(rd) order dispersion of the PSM 20 a is adjusted by using threesections 15, 16, 17 of pulse stretching fiber. The 1st stretcher fiber15 can be a length of fiber with zero 3rd-order and appropriate4^(th)-order dispersion. The 1st stretcher fiber 15 is then spliced tothe 2^(nd) stretcher fiber 16, which is selected to compensate for the3^(rd)-order dispersion of the grating compressor as well as the wholechirped-pulse amplification system. To take advantage of the high3^(rd)-order dispersion of the LP₁₁ mode the 1st stretcher fiber 15 isspliced to the 2^(nd) stretcher fiber 16 with an offset in theirrespective fiber centers, leading to a predominant excitation of theLP₁₁ mode in the 2nd stretcher fiber 16. To maximize the amount of3rd-order dispersion in the 2nd stretcher fiber 16, a fiber with a highnumerical aperture NA>0.20 is preferred. At the end of the 2nd stretcherfiber 16, a similar splicing technique is used to transfer the LP₁₁ modeback to the fundamental mode of the 3^(rd) stretcher fiber 17. By anappropriate choice of fibers, the 4th-order dispersion of the wholeamplifier compressor can be minimized. The 3^(rd) stretcher fiber 17 canbe short with negligible dispersion.

The transfer loss of the whole fiber stretcher assembly is at least 25%due to the unavoidable 50% or greater loss incurred by transferringpower from the LP₁₁ mode to the LP₀₁ mode without the use of opticalmode-converters. Any residual energy in the LP₀₁ mode in the 2ndstretcher fiber can be reflected with an optional reflective fibergrating 18 as shown in FIG. 5. Due to the large difference in effectiveindex between the fundamental and the next higher-order mode, thegrating resonance wavelength varies between 10-40 nm between the twomodes, allowing for selective rejection of one mode versus the other forpulses with spectral widths between 10-40 nm.

The energy loss of the fiber stretcher assembly can be made to beinsignificant by turning the 3^(rd) stretcher fiber 17 into an Ybamplifier. This implementation is not separately shown.

When 4th-order dispersion is not significant, the 1st stretcher fiber 15can be omitted. 4^(th) order dispersion can also be compensated by usinga 1st stretcher fiber with non-zero 3^(rd) order dispersion, as long asthe ratio of 3^(rd) and 4^(th) order dispersion is different between the1^(St) and 2^(nd) stretcher fiber.

The Yb-doped fiber inside the AM1 3 can have an Yb doping level of 2.5mole % and a length of 5 m. Both single-mode and multi-mode Yb-dopedfiber can be used, where the core diameter of the fiber can vary between1-50 μm; though the fundamental mode should be excited in case of a MMfiber to optimize the spatial quality of the output beam. Depending onthe amount of required gain, different lengths of Yb-doped fiber can beused. To generate the highest possible pulse energies, Yb fiber lengthsas short as 1 m can be implemented.

Pulse compression is performed in the PCM 4. The PCM 4 can containconventional bulk optic components (such as the bulk diffraction gratingpair shown in FIG. 4), a single grating compressor, or a number ofdispersive prisms or grisms or any other dispersive delay line.

Alternatively, a fiber or bulk Bragg grating can be used, or a chirpedperiodically poled crystal. The chirped periodically poled crystalcombines the functions of pulse compression and frequency doubling (A.Galvanauskas, et al., ‘Use of chirped quasi-phase matched materials inchirped pulse amplification systems,’ U.S. application Ser. No.08/822,967, the contents of which are hereby incorporated herein byreference) and operates in transmission providing for a uniquely compactsystem.

Other modifications and variations to the invention will be apparent tothose skilled in the art from the foregoing disclosure and teachings.

In particular, the SM 1 can be used as a stand-alone unit to producenear bandwidth limited femtosecond pulses in the frequency range from1.52-2.2 μm, and after frequency conversion in a nonlinear crystal alsoin the frequency range from 760 nm to 1.1 μm. The frequency range can befurther extended by using a fluoride Raman-shifting fiber or otheroptical fibers with infrared absorption edges longer than silica. Usingthis technique wavelengths up to around 3-5 μm can be reached. Inconjunction with frequency-doubling, continuous tuning from 760 nm to5000 nm can be achieved. The pulse power in the 2 μm region can befurther enhanced by using Tm or Ho-doped fiber. With such amplifiers,near bandwidth-limited Raman-solution pulses with pulse energiesexceeding 10 nJ can be reached in single-mode fibers in the 2 μmwavelength region. After frequency-doubling, femtosecond pulses withenergies of several nJ can be obtained in the 1 μm region without theuse of any dispersive pulse compressors. Such pulses can be used as highenergy seed pulses for large-core multi-mode Yb amplifiers, whichrequire higher seed pulse energies than single-mode Yb amplifiers tosuppress amplified spontaneous emission.

An example of an ultra-wide tunable fiber source combining an Er-fiberlaser pulse source 19 with a silica Raman-shifter 20, a Tm-dopedamplifier 21 and a 2^(nd) fluoride glass based Raman shifter 22 is shownin the SM 1 c of FIG. 6. An optional frequency-doubler is not shown; foroptimum stability all fibers should be polarization maintaining. Asanother alternative to the Er-fiber laser pulse source a combination ofa diode-laser pulse source with an Er-amplifier can be used; this is notseparately shown.

As yet another alternative for a SM, SM 1 d is shown in FIG. 7, andcontains a frequency-doubled high-power passively mode-locked Er orEr/Yb-fiber oscillator 23 in conjunction with a length of Raman-shiftingholey fiber 24. Here the pulses from the oscillator 23 operating in the1.55 μm wavelength region are first frequency-doubled using frequencydoubler 25 and lens system 26, and subsequently the frequency-doubledpulses are Raman-shifted in a length of holey fiber 24 that providessoliton supporting dispersion for wavelengths longer than 750 nm or atleast longer than 810 nm. By amplifying the Raman-shifted pulses in the1 μm wavelength regime or in the 1.3, 1.5, or 2 μm wavelength regime andby selecting different designs of Raman-shifting fibers, a continuouslytunable source operating in the wavelength region from around 750 nm to5000 nm can be constructed. The design of such a source with a number ofattached amplifiers 27 is also shown in FIG. 7.

For optimum Raman self-frequency shift, the holey fiber dispersionshould be optimized as a function of wavelength. The absolute value ofthe 3rd order dispersion of the holey fiber should be less than or equalto the absolute value of the 3rd order material dispersion of silica.This will help ensure that the absolute value of the 2nd orderdispersion remains small over a substantial portion of the wavelengthtuning range. Moreover the value of the 2nd order dispersion should benegative, and a 2nd order dispersion zero should be within 300 nm inwavelength to the seed input wavelength.

As yet another alternative for a seed source for an Yb amplifier,anti-Stokes generation in a length of anti-Stokes fiber can be used.After anti-Stokes generation, additional lengths of fiber amplifiers andRaman-shifters can be used to construct a widely wavelength-tunablesource. A generic configuration is similar to the one shown in FIG. 7,where the frequency-doubling means 25 are omitted and the Raman-shiftermeans 24 are replaced with an anti-Stokes generation means. For example,to effectively generate light in the 1.05 μm wavelength regime in ananti-Stokes generation means using an Er fiber laser seed sourceoperating at 1.55 μm, an anti-Stokes generation means in the form of anoptical fiber with small core diameter and a low value of 3^(rd) orderdispersion is optimum. A low value of 3^(rd) order dispersion is heredefined as a value of 3^(rd) order dispersion smaller in comparison tothe value of 3^(rd) order dispersion in a standard telecommunicationfiber for the 1.55 wavelength region. Moreover, the value of the 2^(nd)order dispersion in the anti-Stokes fiber should be negative.

As yet another alternative seed-source for an Yb amplifier, a passivelymodelocked Yb or Nd fiber laser can be used inside the SM. Preferably anYb soliton oscillator operating in the negative dispersion regime can beused. To construct an Yb soliton oscillator, negative cavity dispersioncan be introduced into the cavity by an appropriately chirped fibergrating 29, which is connected to output fiber 36 as shown in FIG. 8;alternatively, negative dispersion fiber such as holey fiber (T. Monroeet al.) can be used in the Yb soliton laser cavity. A SM incorporatingsuch an arrangement is shown as SM 1 e in FIG. 8. Here the Yb fiber 30can be polarization maintaining and a polarizer 31 can be incorporatedto select oscillation along one axis of the fiber (coupling beingaccomplished with lenses 32). For simplicity, the Yb fiber 30 can becladding pumped from the side as shown in FIG. 8. However, a passivelymodelocked Yb fiber laser incorporating conventional single-mode fibercan also be used. Such an arrangement is not separately shown. In FIG.8, SA 28 is used to induce the formation of short optical pulses. Thegrating 35 is used for dispersive control, and as an intra-cavitymirror. The pump diode 33 delivers pump light through V-groove 34.

An arrangement incorporating a holey fiber can be nearly identical tothe system displayed in FIG. 8, where an additional length of holeyfiber is spliced anywhere into the cavity. In the case of incorporatinga holey fiber, the fiber Bragg grating does not need to have negativedispersion; equally the Bragg grating can be replaced with a dielectricmirror.

Most straight-forward to implement, however, is an Yb oscillatoroperating in the positive dispersion regime, which does not require anyspecial cavity components such as negative dispersion fiber Bragggratings or holey fiber to control the cavity dispersion. In conjunctionwith a ‘parabolic’ Yb amplifier (or ordinary Yb amplifier), a verycompact seed source for a high-power Yb amplifier system can beobtained. Such a Yb oscillator with an Yb amplifier 40 is shown in FIG.9, where preferably the Yb amplifier 40 is a ‘parabolic’ Yb amplifier asdiscussed below. Elements which are identical to those in FIG. 8 areidentically numbered.

The SM 1 f in FIG. 9 comprises a side-pumped Yb amplifier 40 asdescribed with respect to FIG. 8, though any other pumping arrangementcould also be implemented. The Yb fiber 44 is assumed to be polarizationmaintaining and a polarizer 31 is inserted to select a singlepolarization state. The fiber Bragg grating 37 has a reflectionbandwidth small compared to the gain bandwidth of Yb and ensures theoscillation of pulses with a bandwidth small compared to the gainbandwidth of Yb. The Bragg grating 37 can be chirped or unchirped. Inthe case of an unchirped Bragg grating, the pulses oscillating insidethe Yb oscillator are positively chirped. Pulse generation or passivemodelocking inside the Yb oscillator is initiated by the saturableabsorber 28. The optical filter 39 is optional and further restricts thebandwidth of the pulses launched into the Yb amplifier 40.

To optimize the formation of parabolic pulses inside the Yb amplifier 40inside the SM 1 f, the input pulses should have a bandwidth smallcompared to the gain bandwidth of Yb; also the input pulse width to theYb amplifier 40 should be small compared to the output pulse width andthe gain of the Yb amplifier 40 should be as high as possible, i.e.,larger than 10. Also, gain saturation inside the Yb amplifier 40 shouldbe small.

As an example of a parabolic amplifier a Yb amplifier of 5 m in lengthcan be used. Parabolic pulse formation is ensured by using a seed sourcewith a pulse width of around 0.2-1 ps and a spectral bandwidth on theorder of 3-8 nm. Parabolic pulse formation broadens the bandwidth of theseed source to around 20-30 nm inside the Yb amplifier 40, whereas theoutput pulses are broadened to around 2-3 ps. Since the chirp insideparabolic pulses is highly linear, after compression pulse widths on theorder of 100 fs can be obtained. Whereas standard ultrafast solid stateamplifiers can tolerate a nonlinear phase shift from self-phasemodulation only as large as pi (as well known in the state of the art),a parabolic pulse fiber amplifier can tolerate a nonlinear phase shiftas large as 10*pi and higher. For simplicity, we thus refer to a largegain Yb amplifier as a parabolic amplifier. Parabolic amplifiers obeysimple scaling laws and allow for the generation of parabolic pulseswith spectral bandwidths as small as 1 nm or smaller by an appropriateincrease of the amplifier length. For example, a parabolic pulse with aspectral bandwidth of around 2 nm can be generated using a parabolicamplifier length of around 100 m.

Since a parabolic pulse can tolerate large values of self-modulation anda large amount of spectral broadening without incurring any pulse breakup, the peak power capability of a parabolic amplifier can be greatlyenhanced compared to a standard amplifier. This may be explained asfollows. The time dependent phase delay Φ_(nl)(t) incurred by self-phasemodulation in an optical fiber of length L is proportional to peakpower, i.e.

Φ_(nl)(t)=γP(t)L,

where P(t) is the time dependent peak power inside the optical pulse.The frequency modulation is given by the derivative of the phasemodulation, i.e., δω=γL[∂P(t)/∂t]. For a pulse with a parabolic pulseprofile P(t)=P₀[1−(t/t₀)²], where (−t₀<t<t₀), the frequency modulationis linear. It may then be shown that indeed the pulse profile also staysparabolic, thus allowing the propagation of large peak powers with onlya resultant linear frequency modulation and the generation of a linearpulse chirp.

The chirped pulses generated with the Yb amplifier 40 can be compressedusing a diffraction grating compressor as shown in FIG. 4.Alternatively, a chirped periodically poled crystal 42 and lenses 41could be used for pulse compression as also shown in FIG. 9. Inconjunction with the SM 1 f shown in FIG. 9 a very compact stand-alonesource of femtosecond pulses in the green spectral region around 530 nmcan be obtained.

In addition to the passively modelocked Yb fiber laser 44 shown in FIG.9, alternative sources could also be used to seed the Yb amplifier.These alternative sources can comprise Raman-shifted Er or Er/Yb fiberlasers, frequency-shifted Tm or Ho fiber lasers and also diode laserpulse sources. These alternative implementations are not separatelyshown.

In FIG. 10 a fiber delivery module (FDM) 45 is added to the basic systemshown in FIG. 1. The PSM 2 is omitted in this case; however, to expandthe peak power capability of the amplifier module a PSM 2 can beincluded when required. The Yb amplifier 7 shown in FIG. 10 can beoperated both in the non-parabolic or the parabolic regime.

In its simplest configuration, the FDM 45 consists of a length ofoptical fiber 46 (the delivery fiber). For a parabolic amplifier, thedelivery fiber 46 can be directly spliced to the Yb amplifier 7 withoutincurring any loss in pulse quality. Rather, due to the parabolic pulseprofile, even for large amounts of self-phase modulation, anapproximately linear chirp is added to the pulse allowing for furtherpulse compression with the PCM 4. The PCM 4 can be integrated with theFDM 45 by using a small-size version of the bulk diffraction gratingcompressor 14 shown in FIG. 4 in conjunction with a delivery fiber. Inthis case the delivery fiber in conjunction with an appropriatecollimating lens would replace the input shown in FIG. 4. A separatedrawing of such an implementation is not shown. However, the use of thePCM 4 is optional and can for example be omitted, if chirped outputpulses are required from the system. In conjunction with a PCM 4, thesystem described in FIG. 10 constitutes a derivative of a chirped pulseamplification system, where self-phase modulation as well as gain isadded while the pulse is dispersively broadened in time. The addition ofself-phase modulation in conventional chirped pulse amplificationsystems typically leads to significant pulse distortions after pulsecompression. The use of parabolic pulses overcomes this limitation.

Advanced fiber optic communication systems can also be interpreted aschirped pulse amplification systems (see, e.g., D. J. Jones et al.,IEICE Trans. Electron., E81-C, 180 (1998)). Clearly, the minimization ofpulse distortions by parabolic pulses is equally relevant in opticalcommunication systems.

To obtain pulse widths shorter than 50 fs, the control of third orderand higher-order dispersion in a FDM module or in an optional PSMbecomes significant. The control of higher-order dispersion with a PSMwas already discussed with reference to FIGS. 1 and 5; the control ofhigher-order dispersion in a FDM is very similar and discussed with anexemplary embodiment of the FDM 45 a shown in FIG. 11. Just as in FIG.1, the large third-order dispersion of a W-fiber can be used tocompensate for the third-order dispersion of a bulk PCM 4. Just as inFIG. 5, by using fibers 15, 16, 17 with different values forhigher-order dispersion in the FDM, the higher order dispersion of thewhole system including a PCM 4 consisting of bulk diffraction gratingsmay be compensated.

Alternative embodiments of PSMs are shown in FIGS. 12 and 13, which arealso of practical value as they allow the use of commercially availablelinearly chirped fiber Bragg gratings in the PSM, while compensating forhigher-order dispersion of a whole chirped-pulse amplification systemcomprising PSM as well as PCM. As another alternative, nonlinearlychirped fiber Bragg gratings can also be used in the PSM to compensatefor the dispersion of the PCM. Such an arrangement is not separatelyshown.

To avoid the use of W-fibers or the LP₁₁ mode in the PSM, an alternativeembodiment of a PSM as shown in FIG. 12 is shown as PSM 2 b. Here anegatively linearly chirped Bragg grating 47 is used in conjunction witha single-mode stretcher fiber 48 with negative third-order dispersionand circulator 49. The introduction of the negative linearly chirpedBragg grating increases the ratio of (3^(rd)/2^(nd))-order dispersion inthe PSM 2 b, allowing for the compensation of the high value of 3^(rd)order dispersion in the PCM 4, when a bulk diffraction gratingcompressor is used. The PSM 2 b can also contain W-fibers in conjunctionwith a linearly chirped fiber Bragg grating to further improve theflexibility of the PSM.

As yet another alternative embodiment of a PSM for the compensation ofhigher-order dispersion the arrangement in FIG. 13 is shown as PSM 2 c,comprising a positively linearly chirped fiber Bragg grating 49,circulator 50 and another fiber transmission grating 51. Here thepositively linearly chirped fiber Bragg grating 49 produces positive 2ndorder dispersion and the other fiber transmission grating 51 produces anappropriate amount of additional 2^(nd) 3^(rd) and 4^(th) orderdispersion, to compensate for the linear and higher order dispersioninside the PCM module. More than one fiber transmission grating or fiberBragg grating can be used to obtain the appropriate value of 3^(rd) and4^(th) and possibly even higher-order dispersion.

To increase the amplified pulse energy from an Yb amplifier to the mJrange and beyond, pulse picking elements and further amplificationstages can be implemented as shown in FIG. 14. In this case, pulsepickers 52 are inserted in between the PSM 2 and the 1^(st) amplifiermodule AM1 3 a, as well as between the 1st amplifier stage AM1 3 a and2^(nd) amplifier stage AM2 3 b. Any number of amplifiers and pulsepickers can be used to obtain the highest possible output powers, wherethe final amplifier stages preferably consist of multi-mode fibers. Toobtain a diffraction limited output the fundamental mode in thesemulti-mode amplifiers is selectively excited and guided using well-knowntechniques (M. E. Fermann et al., U.S. Pat. No. 5,818,630). The pulsepickers 52 are typically chosen to consist of optical modulators such asacousto-optic or electro-optic modulators. The pulse pickers 52down-count the repetition rate of the pulses emerging from the SM 1 by agiven value (e.g. from 50 MHz to 5 KHz), and thus allow the generationof very high pulse energies while the average power remains small.Alternatively, directly switchable semiconductor lasers could also beused to fix the repetition rate of the system at an arbitrary value.Further, the pulse pickers 52 inserted in later amplifier stages alsosuppress the build up of amplified spontaneous emission in theamplifiers allowing for a concentration of the output power inhigh-energy ultra-short pulses. The amplification stages are compatiblewith PSMs and PCMs as discussed before; where the dispersion of thewhole system can be minimized to obtain the shortest possible pulses atthe output of the system.

Amplifier module AM1 3 a can be designed as a parabolic amplifierproducing pulses with a parabolic spectrum. Equally, the parabolicpulses from AM1 3 a can be transformed into pulses with a parabolicpulse spectrum in a subsequent length of pulse-shaping or pulsestretching fiber 53 as also shown in FIG. 14, where the interaction ofself-phase modulation and positive dispersion performs thistransformation. This may be understood, since a chirped pulse with aparabolic pulse profile can evolve asymptotically into a parabolic pulsewith a parabolic spectrum in a length of fiber. The parabolic pulseshape maximizes the amount of tolerable self-phase modulation in thesubsequent amplification stages, which in turn minimizes the amount ofdispersive pulse stretching and compression required in the PSM 2 andPCM 4. Equally, parabolic pulse shapes allow the toleration ofsignificant amounts of self-phase modulation in the PSM 2 withoutsignificant pulse distortions.

Once the pulses are stretched, the detrimental influence of self-phasemodulation in subsequent amplifiers can be minimized by using flat-toppulse shapes. A flat-top pulse shape can be produced by inserting anoptional amplitude filter 54 as shown in FIG. 14 in front of the lastamplifier module to produce a flat-top pulse spectrum. A flat-topspectrum is indeed transformed into a flat-top pulse after sufficientpulse stretching, because there is a direct relation between spectralcontent and time delay after sufficient pulse stretching. It can beshown that even values of self-phase modulation as large as 10*π can betolerated for flat-top pulses without incurring significant pulsedistortions.

An amplitude filter as shown in FIG. 14 may in turn also be used tocontrol the amount of higher-order dispersion in the amplifier chain forstrongly chirped pulses in the presence of self-phase modulation whenreshaping of the pulse spectrum in the amplifier can be neglected, i.e.,outside the regime where parabolic pulses are generated. In this caseself-phase modulation produces an effective amount of higher-orderdispersion of:

${\beta_{n}^{SPM} = \left. {\gamma \; P_{0}L_{eff}\frac{^{n}{S(\omega)}}{\omega^{n}}} \right|_{\omega = 0}},$

where P₀ is the peak power of the pulse and S(ω) is the normalized pulsespectrum. L_(eff) is the effective nonlinear lengthL_(eff)=[exp(gL)−1]/g, where L is the amplifier length and g is theamplifier gain per unit length. Thus by accurately controlling thespectrum of strongly chirped pulses with an amplitude filter as shown inFIG. 14, any amount of higher-order dispersion can be introduced tocompensate for the values of higher-order dispersion in a chirped pulseamplification system. It can indeed be shown for 500 fs pulses stretchedto around 1 ns, a phase shift of ≈10π is sufficient to compensate forthe third-order dispersion of a bulk grating compressor (as shown inFIG. 4) consisting of bulk gratings with 1800 grooves/mm. Attractivewell-controllable amplitude filters are for example fiber transmissiongratings, though any amplitude filter may be used to control the pulsespectrum in front of such a higher-order dispersion inducing amplifier.

As another embodiment for the combination of an amplifier module with apulse picker, the configuration displayed in FIG. 15 can be used. Sincevery high energy pulses require large core multi-mode fibers for theiramplification, the control of the fundamental mode in a single-passpolarization maintaining fiber amplifier may be difficult to accomplish.In this case, it may be preferred to use a highly centro-symmetricnon-polarization maintaining amplifier to minimize mode-coupling and toobtain a high-quality output beam. To obtain a deterministicenvironmentally stable polarization output from such an amplifier, adouble-pass configuration as shown in FIG. 15 may be required. Here asingle-mode fiber 55 is used as a spatial mode filter after the firstpass through the amplifier 56; alternatively, an aperture could be usedhere. The spatial mode filter 55 cleans up the mode after the first passthrough the multi-mode amplifier 56, and also suppresses amplifiedspontaneous emission in higher-order modes that tends to limit theachievable gain in a multi-mode amplifier. Lenses 60 can be used forcoupling into and out of amplifier 56, spatial mode filter 55, and pulsepickers 52 a and 52 b. The Faraday rotator 57 ensures that the backwardpropagating light is polarized orthogonal to the forward propagatinglight; the backward propagating light is coupled out of the system atthe shown polarization beamsplitter 58. To optimize the efficiency ofthe system, a near-diffraction limited source is coupled into thefundamental mode of the multi-mode fiber 56 at the input of the system,where gain-guiding can also be used to further improve the spatialquality of the beam amplified in the multi-mode fiber. To count-down therepetition rate of the pulse train delivered from a SM and to suppressamplified spontaneous emission in the multi-mode amplifier, a 1stoptical modulator 52 a can be inserted after the first pass through themulti-mode amplifier. An ideal location is just in front of thereflecting mirror 59 as shown. As a result a double-pass gain as largeas 60-70 dB could be obtained in such a configuration, minimizing thenumber of amplification stages required from amplifying seed pulses withpJ energies up to the mJ energy level. This type of amplifier is fullycompatible with the SMs, PSMs and PCMs as discussed before, allowing forthe generation of femtosecond pulses with energies in the mJ regime. Asanother alternative for the construction of a high-gain amplifiermodule, a count-down of the repetition rate from a pulse train deliveredby a SM can also be performed with an additional 2nd modulator 52 bprior to injection into the present amplifier module as also shown inFIG. 15. The repetition rate of transmission windows of the 1stmodulator 52 a should then be either lower or equal to the repetitionrate of the transmission window of the 2nd modulator 52 b. Such aconfiguration is not separately shown. FIG. 15 shares some similaritieswith FIG. 5 of U.S. Pat. No. 5,400,350, which is hereby incorporated byreference.

As yet another alternative embodiment of the present invention, anoptical communication system using the formation of parabolic pulses inlong, distributed, positive dispersion amplifiers 61 is shown in FIG.16. Dispersion compensation elements 63 are inserted in-between thefiber optic amplifiers. Optical filters 62 are further implemented tooptimize the pulse formation process in the amplifiers. The opticalfilters can be based on optical etalons with a limited free spectralrange, so as to produce a spectrally repetitive transmissioncharacteristic, allowing for the simultaneous transmission of multiplewavelength channels as required for wavelength-division multiplexing.

The key benefit is the combination of large amounts of gain in longlengths of positive dispersion fiber, to linearize the chirp introducedby optical Kerr-nonlinearities in the fiber transmission system.Therefore, generally, the transmission characteristics of an opticalcommunication system are improved by implementing positive dispersion(non-soliton supporting) amplifiers. Such amplifiers can have lengths ofat least 10 km, and a gain of less than 10 dB/km. The total gain peramplifier can exceed 10 dB, in order to exploit the onset of parabolicpulse formation for a minimization of the deleterious effect of opticalnonlinearities. Further improvements are gained by using amplifiershaving a gain of less then 3 dB/km, and increasing the total length sothat the total gain is greater than 20 dB. A still further improvementin the transmission characteristics of the fiber transmission line isobtained by minimizing the amount of Kerr-nonlinearities in the negativedispersion elements of the fiber transmission line. This can beaccomplished by the use of chirped fiber gratings for the negativedispersion elements.

In addition to forming parabolic pulses inside the transmission line, itcan also be beneficial to generate the parabolic pulses in an externalsource, and then to inject them into non-soliton supporting amplifierfiber. To make effective use of such a system, low-loss positivedispersion transmission as enabled by holey fibers is useful. Along thefiber transmission line and at the end of the fiber transmission line,dispersion compensating elements are implemented. Such a systemimplementation is similar to the one shown in FIG. 16, and is notseparately shown.

An optical communication system designed along similar lines asdescribed above is disclosed in Provisional Application No. 60/202,826,now U.S. application Ser. No. 10/275,137, which is hereby incorporatedherein by reference.

As yet another embodiment of the present invention in thetelecommunications arena, a wavelength tunable Raman amplifier can beconstructed using Raman-shifted pulses. It is well known in the state ofthe art that a high-power optical signal at a given pump wavelengthproduces Raman gain at a signal wavelength which is red-shifted withrespect to the pump wavelength. In fact, it is this effect acting uponthe pump pulse itself which is used for the construction of thewavelength-tunable pulse sources discussed herein.

A generic design for a wavelength-tunable Raman amplifier is shown inFIG. 17. Here short optical pulses are generated in a seed source 64.The seed pulses are optically modulated by modulator 65 and can also beamplified in an optical amplifier 66. The seed pulses are then injectedinto a length of Raman-shifting fiber 67. The Raman-shifting fiber canbe a length of holey fiber or of any other design. The time periodbetween the Raman-shifted pulses can be reduced by using a pulsesplitting means (pump pulse splitter) 68 as shown in FIG. 17. This pulsesplitting means could for example be an array of imbalanced Mach-Zehnderinterferometers, though any means for generating a pulse train from asingle pulse is acceptable. The appropriately wavelength-shifted,amplified and modulated seed pulses comprise the pump pulses that areinjected into the Raman amplifier fiber 69 and generate optical gain ata signal wavelength inside the Raman amplifier as shown in FIG. 17, tothereby operate on signal in 70 to produce signal out 71.

Inside the Raman amplifier fiber the optical signal at the signalwavelength is counter propagating with respect to the pump pulses in theRaman amplifier. Also several signal wavelengths can be simultaneouslyinjected into the Raman amplifier using a signal combiner, making suchan amplifier compatible with optical wavelength division multiplexing.For example, pump pulses at a wavelength of 1470 nm generate Raman gainaround the 1560 nm wavelength region in a silica fiber. To optimize thegain of the Raman amplifier, holey fiber or other fiber with arelatively small fiber core diameter can be used.

The center wavelength of the wavelength at which Raman gain is obtainedis then tunable by tuning the wavelength of the pump pulses.Wavelength-tuning of the pump pulses can be accomplished by modulatingthe power as well as the width of the seed pulses before injected theminto the Raman-shifter fiber 67.

Moreover, the gain spectrum of the Raman amplifier can be adjusted byrapidly tuning the wavelength of the pump pulses, such that the signalpulses are subjected to an effective modified Raman gain spectrum. Tomake sure the effective Raman gain is independent of time, the speed oftuning the pump pulses, i.e. the time period it takes to tune the pulsesacross a desired wavelength range, should be small compared to the timeit takes for the signal pulses to traverse the Raman amplifier fiber 69.

Thus for Raman amplifiers for telecomm systems it is advantageous toobtain broader spectral gain than is possible from a single pulse. It isalso advantageous to be able to dynamically change the gain in WDMtelecommunication systems to compensate for the varying amount of databeing transmitted at different wavelengths. The one way to broaden thespectral gain is to rapidly tune the pump wavelength compared to thepropagation time through the communication fiber. The gain can bedynamically adjusted by varying the time that the pump remains atdifferent wavelengths. An alternative means of adjusting the gainspectrum is to use a plurality of pump pulses into the Raman shiftingfiber each at a different wavelength. Modulating the relative number ofpulses at each of the wavelengths can then modify the relative gainprofile.

More specifically, the femtosecond pulse source described in FIG. 1 isamplified in Yb amplifiers to high powers. These pulses can then beRaman self-frequency shifted to the 1400-1500 nm range by a fiber withthe zero dispersion point at a shorter wavelength than the operatingpoint of the femtosecond pulse source. This fiber could be a holeyfiber. In order to attain power in the watt level with the Ramanself-frequency shift to the 1400-1500 nm range, the optimum repetitionrate of the source will be at higher frequencies, such as greater than 1Ghz. Gain spectral broadening and automatic gain control can be obtainedby using a plurality of pump wavelengths, by tuning the pump wavelengthor by modulating the pulse amplitude of individual pulses in the pulsetrain to obtain different amounts of Raman shift.

What is claimed is:
 1. A passively modelocked fiber laser, comprising: alaser cavity comprising at least one Yb or Nd doped gain fiber; and apump diode to pump said laser cavity, wherein said laser cavity isconfigured for operation in a positive dispersion regime withoutnegative dispersion cavity components.
 2. The passively modelocked fiberlaser according to claim 1, further comprising a saturable absorberdisposed in said laser cavity to initiate mode locking.
 3. The passivelymodelocked fiber laser according to claim 1, wherein said doped gainfiber is a Yb doped gain fiber configured as polarization maintainingfiber.
 4. The passively modelocked fiber laser according to claim 1,wherein said cavity comprises a polarizer to select a polarizationstate.
 5. The passively modelocked fiber laser according to claim 1,wherein said pump diode and said Yb or Nd doped gain fiber are arrangedfor cladding pumping.
 6. The passively modelocked fiber laser accordingto claim 1, wherein said laser cavity comprises an unchirped fiber Bragggrating, and wherein pulses oscillating in said cavity are positivelychirped.
 7. The passively modelocked fiber laser of claim 1, whereinsaid cavity is configured such that an oscillation bandwidth ofintracavity pulses is small compared to the gain bandwidth of Yb.
 8. Thepassively modelocked fiber laser or claim 1, wherein said cavitycomprises a fiber grating arranged as a cavity end mirror, said fibergrating having a reflection bandwidth smaller than a gain bandwidth ofsaid Yb or Nd doped gain fiber.
 9. An optical pulse source, comprising:a passively modelocked fiber laser according to claim 1, and a Ybamplifier disposed downstream from said passively modelocked fiber laserand configured to amplify an output of said passively mode locked fiberlaser.
 10. The optical pulse source according to claim 9, wherein saidYb amplifier is configured to generate parabolic pulses.
 11. The opticalpulse source according to claim 9, further comprising a saturableabsorber disposed in said fiber laser cavity to initiate mode locking.12. A passively mode locked fiber laser, comprising: a laser cavitycomprising at least one Yb or Nd doped gain fiber; a pump diode to pumpsaid laser cavity; and a negative dispersion fiber configured to controlintra-cavity dispersion inside the cavity.
 13. A passively mode lockedfiber laser according to claim 12, wherein said negative dispersionfiber comprises holey fiber.
 14. A passively mode locked fiber laseraccording to claim 13, wherein said holey fiber is spliced to at leastone fiber component in said cavity.
 15. A passively mode locked fiberlaser according to claim 12, wherein said cavity comprises a length ofpolarization maintaining fiber.
 16. A passively mode locked fiber laseraccording to claim 12, further comprising a saturable absorber toinitiate modelocking.
 17. A passively mode locked fiber laser accordingto claim 12, further comprising a polarizer.
 18. The passivelymodelocked fiber laser according to claim 12, wherein said pump diodeand said Yb or Nd doped gain fiber are arranged for cladding pumping.19. An optical pulse source, comprising: a passively modelocked fiberlaser according to claim 12, and a Yb amplifier disposed downstream fromsaid passively modelocked fiber laser and configured to amplify anoutput of said passively mode locked fiber laser.
 20. A laser systemproducing output in the 3-5 μm wavelength region, comprising: afemtosecond pulse source; a fiber having an infrared absorption edgelonger than that of silica.
 21. A tunable laser system producingwavelength tunable pulses, comprising: a seed module producingfemtosecond pulses in the spectral range from about 1.5-2.2 μm; and aninfrared transmitting fiber having an infrared absorption edge longerthan that of silica, said infrared transmitting fiber capable offrequency shifting pulses produced with said seed module to longerwavelengths, up to the spectral range from about 3-5 μm.
 22. A lasersystem comprising: a fiber laser source generating pulses in the 1-1.15um wavelength region which have a spectral bandwidth larger than 0.3 nmand a pulse width between 50 fs and 1 ns, said laser system comprising:a frequency-shifting fiber which outputs an anti-Stokes, blue-shiftedoutput.
 23. An optical pulse source comprising: a diode-pumped passivelymodelocked fiber laser including a saturable absorber and capable ofgenerating femtosecond pulses in the 1-1.15 μm wavelength region; a Yb:fiber amplifier for amplifying an output of said fiber laser; and anoptical modulator disposed between said passively modelocked fiber laserand said Yb: amplifier, said modulator being controllable to reduce therepetition rate of pulses emitted from said modelocked fiber laser, toincrease the available pulse energy of pulses amplified with said Yb:amplifier, and to provide for selection of pulses for amplification withsaid Yb: amplifier.
 24. A fiber-based chirped pulse amplification systemcomprising: the optical pulse source according to claim 23; and acombination of a pulse stretcher and pulse compressor optically coupledto said optical pulse source.
 25. A laser system comprising: a seedsource generating pulses in the 1-1.15 μm wavelength region which have aspectral bandwidth larger than 0.3 nm and a pulse width betweenapproximately 50 fs and 1 ns; a cladding-pumped fiber amplifier forbroad bandwidth pulses which receives, amplifies and outputs saidpulses, wherein the fiber amplifier is operated with at least oneforward and one backward pass; and a pump laser for providing laserenergy to said fiber amplifier, and an optical modulator located betweenthe one forward and one backward pass of said amplifier.
 26. A lasersystem according to claim 25, further comprising: a plurality ofadditional fiber amplifiers, wherein at least one of the fiber amplifierand the plurality of additional fiber amplifiers is operated with atleast one forward and one backward pass.
 27. A laser system according toclaim 26, further comprising at least one pulse picker located betweenthe at least one forward and one backward pass.
 28. A laser systemaccording to claim 25, wherein the seed source produces pulses whichinduce the formation of parabolic pulses within said fiber amplifier.29. A laser system according to claim 25, further comprising: a couplerbetween the seed source and the fiber amplifier, which couples the seedsource to the fiber amplifier, and which further comprises an opticalfiber with a length less than 1 km.
 30. A laser system according toclaim 25, further comprising: an optical delivery fiber coupled to theoutput of the fiber amplifier.
 31. A laser system according to claim 30,wherein said optical delivery fiber is selected from the groupconsisting of: a holey fiber, a length of few-moded fiber and a lengthof few-moded fiber spliced together with one or two lengths ofsingle-mode fiber.